1. Technical Field
The present invention relates to surface casting or overmolding lens casting onto a photochromic polycarbonate substrate and a method to achieve same.
2. Description of Related Art
Optical lenses are typically manufactured from polycarbonate due to the myriad of advantages such material offers which make it ideally suited for optical use. Namely, polycarbonate possesses a high index of refraction, blocks all UVA and UVB radiation, is an ideal candidate for injection molding processes, is easily drilled (e.g., for securing to eye frames), and is lightweight and highly impact resistant.
Some polycarbonate lenses further include photochromic materials or additives. Photochromic lenses change from light to dark when exposed to ultraviolet (UV) radiation. Typically, molecules of the active ingredient in the photochromic additive that causes the lenses to transform are incorporated evenly throughout the lens material. However, in instances where a particularly strong prescription lens is made, the thickest parts of the lens will become darker upon exposure to UV light than the thinner portions. In addition, if there is a large difference in prescription between the two eyes of a wearer, the lens with the stronger prescription will become darker upon exposure to UV light than the weaker one.
To achieve a uniform color density throughout the lenses regardless, e.g., of the lens prescription and/or lens thickness, photochromic coatings have been developed which are added as a layer or coating onto the surface of the lens. Such coatings result in an even darkening of the overall lens regardless of lens prescription or thickness.
However, polycarbonate coated with photochromic additives is relatively delicate and vulnerable to scratches and other surface insults. When a lens is scratched or otherwise damaged, the optical properties of the lens are negatively affected. Various methodologies for imparting scratch resistance to polycarbonate lenses are known, among them the application or integration of scratch-resistant coatings to the lens itself.
For users with, e.g., presbyopia, lenses must be designed to offer more than one focal point or lens power (multifocal lenses) within each lens. Such lenses are often configured such that the area of the lens that offers a separate focal distance is segmented and projects away from at least one major optical surface of the lens. Such segmented lenses may include, e.g., a raised or protruding area (segment) which is elevated from the rest of the lens area, and thus have a segment line or ridge separating the powers. Segmented lenses thus have a non-uniform surface profile. Straight top bifocal lenses are one example, having a “D” shaped segment with a top straight protruding edge dedicated to one focal distance.
One method for producing ophthalmic lenses involves the casting of a surface onto an optical wafer, or preform, to form a lens. Advantageously, the casting of a surface onto an optical perform can reduce the number of molds required to produce a full prescriptive range of lenses and thus is cost-effective. However, successful and secure adhesion of the casting surface onto the lens is often a problem. In particular, successful adhesion of a surface onto a lens having a photochromic coating is especially difficult to achieve.
The challenge thus far is to produce a uniformly photochromic segmented lens in a cost effective and most efficient manner that exhibits excellent durability and scratch resistance. Indeed, using current viscous photochromic coating deposition techniques, it is extremely difficult, if not impossible, to produce a durable polycarbonate photochromic semi-finished segmented (multifocal) optical lens.
U.S. Pat. No. 5,531,940 describes various methods for making optical plastic lenses with photochromic additives. U.S. Pat. No. 5,702,819 describes methods for producing scratch resistant lenses inexpensively. U.S. Pat. No. 6,455,653 describes a composition suitable for use as a cast layer over a preform. U.S. Pat. No. 6,630,083 discusses methods and compositions for producing lenses by casting, namely, via a two-stage ultraviolet cure. Australian Patent No. 80556/87 describes a casting method for a polymeric lens substrate which involves the roughening of the surface of the lens so as to promote subsequent adhesion of the casting material.
Photochromic (PhCh) semi-finished straight top (SFST) lenses have be made by injection molding polycarbonate (PC) behind a photochromic wafer. The wafer is typically a tri-layer of PC/PhCh/PC. However, while injection molding may provide a photochromic polycarbonate lens, it requires a significant investment in costly machinery.
Accordingly, an efficient and effective technique for producing polycarbonate photochromic lenses, in particular photochromic segmented lenses is highly desirable.